Hi,i've seen the world record run and i was wondering how i can obtain that speed in game

You need to move forward with the keyboard and mouse at the same time in order to achieve the top speed

Technically the fastest way to move is (I think) keyboard, mouse, run and strafe together - remember you can't turn and strafe together though (on the OG at least).

Testing with DOSBox confirms that you can mouserun faster by increasing the mouse sensitivity.